AlliedMods Answer

Well, what's the problem then? You have TcpListener and TcpClient in C#. You have a port onto where to listen for what you want and you can send back some data. Let me give you an example and be warned, I'm not a C# expert, I just have some knowledge of it.

Here we have the server code:
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namespace Server
{
    class Program
    {
        const int PORT = 5000;
        const string HOST = "127.0.0.1";

        static void Main(string[] args)
        {
            /*! Listen at the specified host and port number */
            IPAddress hostAddress = IPAddress.Parse(HOST);
            TcpListener listener = new TcpListener(hostAddress, PORT);
            Console.WriteLine("Listening...");
            listener.Start();

            /*! Accept incoming client *//
            TcpClient client = listener.AcceptTcpClient();

            /*! Get the incoming data through a network stream */
            NetworkStream networkStream = client.GetStream();
            byte[] buffer = new byte[client.ReceiveBufferSize];

            /*! Read the data from the network stream */
            int bytesRead = networkStream.Read(buffer, 0, client.ReceiveBufferSize);

            /*! Convert the received data into string and print it out */
            string dataReceived = Encoding.ASCII.GetString(buffer, 0, bytesRead);
            Console.WriteLine("Received : " + dataReceived);

            /*! Return a response to the client */
            Console.WriteLine("Sending back: " + dataReceived);
            networkStream.Write(buffer, 0, bytesRead);
            client.Close();
            listener.Stop();
            Console.ReadLine();
        }
    }
}
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And we have a client (again in C#) just so you are able get the idea how the transmission of data through network actually works:

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namespace Client
{
    class Program
    {
        const int PORT = 5000;
        const string HOST = "127.0.0.1";
        static void Main(string[] args)
        {
            /*! Data to send to the server */
            string message = DateTime.Now.ToString();

            /*! Create a TCPClient object to connect to the host with port*/
            TcpClient client = new TcpClient(HOST, PORT);
            NetworkStream networkStream= client.GetStream();
            byte[] bytesToSend = ASCIIEncoding.ASCII.GetBytes(message);

            /*! Send the message */
            Console.WriteLine("Sending: " + message);
            networkStream.Write(bytesToSend, 0, bytesToSend.Length);

            //---read back the text---
            byte[] bytesToRead = new byte[client.ReceiveBufferSize];
            int bytesRead = networkStream.Read(bytesToRead, 0, client.ReceiveBufferSize);
            Console.WriteLine("Received: " + Encoding.ASCII.GetString(bytesToRead, 0, bytesRead));
            Console.ReadLine();
            client.Close();
        }
    }
}
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You can learn much more onto how the HTTP servers and listeners work overall at the RFC2616 HTTP/1.1 - https://www.ietf.org/rfc/rfc2616.txt